| Brown carbon?BrC?is a new type of organic carbon?OC?that absorbs light radiation in the near ultraviolet and visible regions in the past decede.It not only causes radiative forcing to the atmosphere,but also plays an important role in the rate of atmospheric photochemical reactions.The emergence of BrC breaks the inherent concept of dividing carbonaceous aerosols into two components,black carbon?BC?with strong light absorption and organic carbon without light absorption,and introduces uncertainty for the assessment of climate models,transmission models and emission inventories.Atmospheric BrC is affected by different sources,atmospheric processes,formation mechanisms,etc.,thus,our understanding of the relationship between the chemical composition and optical properties of BrC remains limited,which has become one of research hotpots.Biomass burning,coal combustion and vehicle exhaust are important sources of BrC.It is very important for evaluting atmospheric evolution of BrC by studying its optical properties,chemical structure,and molecular composition in different emission sources.Thus,this article contains four parts to discuss the characteristics of soluble BrC in typical sources,and further apply it to the source apportionment of BrC in atmospheric aerosols.The first part of this paper analyzes the water-soluble organic carbon?WSOC?and methanol-soluble organic carbon?MSOC?in the aerosol particles emitted from simulated IndoChina Peninsula biomass burning,and residental coal combustion,and vehicle emissions?including tunnel aerosol and vehicle exhaust particulates?using UV-vis absorption spectscopy,excitation-emission matrix?EEM?fluorescence spectscopy and Fourier transform ion cyclotron resonance mass spectrometry?FT-ICR MS?.The results indicated that WSOC in biomass and coal combustion aerosols has a strong light absorption capacity,which was lower than that of MSOC.Combination of EEM and parallel factor?PARAFAC?analysis determined two kinds of HULIS?humic-like substances,P1 and P6?,three types of PLOM?Protein-like substances,P2,P3 and P5?and an undefined component?P4?in WSOC,of which depends on the different types of emission sources.For example,HULIS are abundant in tunnel aerosol samples;biomass burning aerosols have higher P2 component;coal combustion and vehicle emissions have higher P4 component;and primary vehicle exhaust particles have higher P5 component.Similar results were observed in the MSOC.FT-ICR MS results show that the main compounds in WSOC are in the range of m/z 200?400 and the MSOC is mainly in the range of m/z 350?600?except for coal combustion samples?.CHO and CHON compounds are the main components in the aerosol samples.However,WSOC from coal combustion and tunnel aerosol samples contains higher levels of S-containing compounds than biomass burning aerosols and primary vehicle exhaust particles.Van Krevelen show that the compounds in coal combustion samples have higher unsaturation degree and lower oxidation levels,as well as higher aromatic content.The unique molecules identified by the venn diagram show specific chemical characteristics in the van Krevelen,which may be responsible for the different fluorescence characteristics of different sources.The relationship between optical properties and chemical structure indicates that in the source emission samples,the light absorption capacity is associated with the unsaturation degree and molecular weight of these compounds.Our result clarifies the characteristics of BrC absorption and molecular composition in different sources,laying the foundation for the classification and source apportionment of chromophores in atmospheric aerosols using EEM-based and molecular characteristics-based methods.HULIS is important components of BrC.On the basis of the simulated IndoChina Peninsula biamass burning,the second part of this paper calculates the emission factors of HULIS and light absorption,and further evaluates the emissions amount of HULIS in Southeast Asia.UV-vis absorption spectra indicate water-soluble HULIS exhibits strong light absorption,which was affected by combustion conditions.HULIS accounts for 85±10%of light absorption at 365 nm,indicating HULIS is mainly absorbing component of BrC.Using the carbon balance formula,emission factors?EFs?of HULIS was calculated with value of 2.3±2.1 g kg-1 fuel.Four vegetation subtypes burning emitted different HULIS,inferring that there was large uncertainty when only a single EFs that represents all types of tropical forest vegetation was used to evaluate HULIS amount.Based on the calculated EFs of HULIS,burned area of Southeast Asia,aboveground biomass density and combustion efficiency,the annual emissions of HULIS in Southeast Asia in 2016 was 223 Gg with an uncertainty of 30%.Cambodia has the highest emissions,with a total of 93 Gg throughout the year,followed by Burma?79 Gg?and Thailand?32 Gg?.The seasonal distribution indicates that the emissions from January to April in Southeast Asia account for 99%of the total annual emissions of HULIS.HULIS emitted from biomass burning are large and its strong light absorption indicate that they have an important impact on regional radiation and climate change.BrC emitted from biomass burning in Southeast Asia is potential to impact on atmospheric radiative forcing.In order to further understand the influence of biomass burning in this region on atmospheric BrC,the third part of this paper selected Bangkok of Thailand for the research,water-soluble and methanol-soluble BrC in aerosol in 2016were investigated using EEM fluorescence spectroscopy and UV-visible absorption spectroscopy.BrC has a strong light absorption in this region,especially in the near UV,which is higher than that of BC.Relationship between light absorption coefficient?Abs?and molecular tracers shows that BrC in aerosol from Bangkok,is mainly related to fossil fuels combustion during the monsoon season?June to October?,and biomass burning during other seasons.Biomass burning activities is closely related to the light absorption of BrC,which is potential to the main factor for the enhancement of light absorption capacity.Combining PARAFAC and source emission fluorescence spectra decomposes the fluorophores in aerosols from Bangkok.Different fluorescence characteristics were observed between aerosol samples and source emission samples.The source emission fluorescence characteristics could help to understand the characteristics of the fluorophores in atmospheric aerosols.Fluorescence indies shows that these soluble organic substances have low humification and are a mixture of terrestrial and microbial organic substances.The Abs of water-soluble and methanol-souble BrC is mainly related to the chromophores with longer emission wavelength,and these groups usually have higher conjugated systems.Overall,on one hand,biomass burning has an impact on atmospheric BrC,which may be the mainly factor for enhancement of BrC absorption,and source emission fluorescence may be not suit for source apportionment for BrC chromophores,because atmospheric process may change the chemical structure of fluorophores;on the other hand,the light absorption of BrC can be attributed to a limited number of strong chromophores.Atmospheric BrC have complex compositions and a wide range of sources,and its classification and identification could help to clarify the spatial and temporal distribution.The fourth part of this article introduces a new method for rapid identification and classification of atmospheric BrC.Fluorescence,origins and molecular characteristic of WSOC in PM2.5 of urban and background sites in five important cities of China were analyzed using EEM-PARAFAC combined principal component analysis?PCA?,stable carbon isotope and FT-ICR MS.The application of EEM-PARAFAC-PCA method achieve the identification and classification of BrC in different regions.Beijing is assigned in a cluster,mainly related to N-containing compounds and HULIS or fulvic-acid with aromatic ring structure.Lanzhou,Wuhan background site and Chengdu urban site are assigned in a cluster,related to HULIS or fulvic acid or PLOM with a low degree of conjugation.Wuhan urban site and Shanghai urban site are in a cluster.Stable carbon isotope results further support the above classification.WSOC in PM2.5 samples in Beijing is mainly related to the coal and liquid fossil fuel.Lanzhou,Wuhan and Chengdu sites are mainly related to the C3 plants and liquid fossil fuels.Shanghai site may be affected by marine aerosols.FT-ICR MS analysis indicate that the differences between urban and background sites in Chengdu,Shanghai,and Wuhan may be caused by the types of different molecular compounds. |
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